Where electrical isolation for dies constituting power devices is required, a layer of dielectric material, such as alumina, may be disposed between the semiconductor die material constituting the device and its associated heatsink. After being formed by rolling from its liquid or green state, and fired the alumina may be clad with copper by a eutectic bonding process and the die soldered to the clad alumina as a substrate. The opposite surface of the alumina may then be soldered in turn to a metal heatsink. Alternatively, the soldered attachments may be achieved by other methods of forming a solderable surface on alumina.
Unfortunately, mismatch between the thermal coefficients of expansion of alumina (6.4 parts per million per degrees C) and the metal of the heatsink such as for example copper (17 parts per million per degree C), limits the size of clad alumina substrate which can be soldered to a heatsink without unacceptable risk of stress damage during thermal cycling. A further problem with large substrates is the difficulty of obtaining a void-free attachment of the soldered surfaces.
The drawbacks of large substrates results in a practical limit of about 1 in. by 1 in. being placed on the substrate. In large circuits, therefore, multiple substrates for example individually attached to a common heatsink are used resulting in more expensive assembly and potentially unreliable interconnections, particularly since, even with mechanized assembly techniques, good registration in the placement of multiple substrates is difficult.
Where such multiple substrates are employed, they may be formed from a common piece of substrate material having grooves scored therein to give fault lines of preferred fracture when subjected to bending. Such material is sometimes referred to in the art as `snap-strate` since in use dies are first soldered to the substrate as a single piece which is then snapped along its grooves to form a plurality of acceptably small substrates for attachment to the heatsink. Any interconnections required between dies are then made, for example by wire bonding.